1. Field of the Invention
This invention relates generally to methods and apparatus for transforming energy from a heat source into usable form using a working fluid that is expanded and regenerated. This invention further relates to a method and apparatus for improving the heat utilization efficiency of a thermodynamic cycle.
2. Brief Description of the Background Art
In the Rankine cycle, a working fluid such as water, ammonia or a freon is evaporated in an evaporator utilizing an available heat source. The evaporated gaseous working fluid is expanded across a turbine to transform its energy into usable form. The spent gaseous working fluid is then condensed in a condenser using an available cooling medium. The pressure of the condensed working medium is increased by pumping, followed by evaporation and so on to continue the cycle.
The Exergy cycle, described in U.S. Pat. No. 4,346,561, utilizes a binary or multi-component working fluid. This cycle operates generally on the principle that a binary working fluid is pumped as a liquid to a high working pressure and is heated to partially vaporize the working fluid. The fluid is then flashed to separate high and low boiling working fluids. The low boiling component is expanded through a turbine, to drive the turbine, while the high boiling component has heat recovered for use in heating the binary working fluid prior to evaporation. The high boiling component is then mixed with the spent low boiling working fluid to absorb the spent working fluid in a condenser in the presence of a cooking medium.
The theoretical comparison of the conventional Rankine cycle and the Exergy cycle demonstrates the improved efficiency of the new cycle over the Rankine cycle when an available, relatively low temperature heat source such as ocean water, geothermal energy or the like is employed.
In applicant's further invention, referred to as the Basic Kalina cycle, the subject of U.S. Pat. No. 4,489,563, relatively lower temperature available heat is utilized to effect partial distillation of at least a portion of a multi-component fluid stream at an intermediate pressure to generate working fluid fractions of differing compositions. The fractions are used to produce at least one main rich solution which is relatively enriched with respect to the lower boiling component, and to produce one lean solution which is relatively impoverished with respect to the lower boiling component. The pressure of the main rich solution is increased; thereafter, it is evaporated to produce a charged gaseous main working fluid. The main working fluid is expanded to a low pressure level to convert energy to usable form. The spent low pressure level working fluid is condensed in a main absorption stage by dissolving with cooling in the lean solution to regenerate an initial working fluid for reuse.
In any process of converting thermal energy to a usable form, the major loss of available energy in the heat source occurs in the process of boiling or evaporating the working fluid. This loss of available energy (known as exergy or essergy) is due to the mismatch of the enthalpy-temperature characteristics of the heat source and the working fluid in the boiler. Simply put, for any given enthalpy the temperature of the heat source is always greater than the temperature of the working fluid. Ideally, this temperature difference would be almost, but not quite, zero.
This mismatch occurs both in the classical Rankine cycle, using a pure substance as a working fluid, as well as in the Kalina and Exergy cycles described above, using a mixture as the working fluid. The use of a mixture as a working fluid in the manner of the Kalina and Exergy cycles reduces these losses to a significant extent. However, it would be highly desirable to further reduce these losses in any cycle.
In the conventional Rankine cycle, the losses arising from mismatching of the enthalpy-temperature characteristics of the heat source and the working fluid would constitute about 25% of the available exergy. With a cycle such as that described in U.S. Pat. No. 4,489,563, the loss of exergy in the boiler due to enthalpy-temperature characteristics mismatching would constitute about 14% of all of the available exergy.
The overall boiling process in a thermodynamic cycle can be viewed for discussion purposes as consisting of three distinct parts: preheating, evaporation, and superheating. With conventional technology, the matching of a heat source and the working fluid is reasonably adequate during preheating. However, the quantity of heat in the temperature range suitable for superheating is generally much greater than necessary, while the quantity of heat in the temperature range suitable for evaporation is much smaller than necessary. The inventor of the present invention has appreciated that a portion of the high temperature heat which would be suitable for high temperature superheating is used for evaporation in previously known processes. This causes very large temperature differences between the two streams, and as a result, irreversible losses of exergy.
These irreversible losses may be lessened by reheating the stream of working fluid after it has been partially expanded in a turbine. However, reheating results in repeated superheating. As a result, reheating increases the necessary quantity of heat for superheating. This increase in the required heat provides better matching between the heat source and the working fluid enthalpy-temperature characteristics. However, reheating has no beneficial effect with respect to the quantity of heat necessary for evaporation. Thus, the total quantity of heat necessary per unit of weight of working fluid significantly increases with reheating. Therefore, the total weight flow rate of working fluid through the boiler turbine is reduced. Thus, the benefits of reheating are largely transitory in that the reduced weight flow rate limits the possible increase in overall efficiency that may be derived.
The ideal solution to the age old dilemma of poorly matched heat source and working fluid enthalpy-temperature characteristics would be one that makes high temperature heat available from the heat source for use in superheating thereby reducing the temperature differences during superheating, but at the same time provides lower temperature heat which minimizes the temperature differences in the process of evaporation. It should be evident that these two goals are apparently mutually inconsistent since increasing the superheating heat would appear to require either increasing the overall heating source temperature or using reheating. As discussed above, reheating has certain drawbacks, which to a large degree mitigate the partly transitory gains achieved.
Moreover, the greater the available heat for superheating, the greater would be the output temperature of the gaseous spent working fluid from the turbine. This is undesirable from an efficiency standpoint since the superheating of the exiting steam makes subsequent condensing more difficult and causes additional losses of exergy. Thus, any effort to improve efficiency with respect to one part of the cycle seems to eventually cause lower efficiency in another part of the cycle.